The invention relates to a nonaqueous electrochemical cell. Such cells have a substantial practical-significance, in particular as electrical batteries. Non-rechargeable batteries (primary cells), as well as rechargeable batteries (secondary cells), have, in many cases, electrochemical cells with nonaqueous electrolytes.
For such cells, the required safety is an important issue. For many cell types, a strong heating, in particular, may lead to safety-critical conditions. It may happen that the cell housing breaks, or leakage occurs, causing noxious gaseous or solid matters, or even fire, to come out of the cell. A quick temperature rise can be caused not only by inadequate treatment, but also by internal or external short circuits during the cell operation.
In particular, such cells in which a strong increase of the temperature inside the cell supports exothermic reactions, are critical because this leads to a further increase of the temperature. This self-amplifying effect is by those skilled in the art also named as xe2x80x9cthermal runwayxe2x80x9d. Such problems are mainly discussed in the context of alkaline metal cells, i.e. cells in which an alkaline metal is deposited or incorporated, respectively, at the electrodes as active mass.
For experimental examination, so-called xe2x80x9cnail testsxe2x80x9d are performed. An internal short-circuit is simulated by piercing the positive and the negative electrode with a nail. For example with lithium ion cells, those tests resulted in a strong temperature increase and a violent discharge of burning and noxious battery components. In practice, such safety problems may occur not only in case of a mechanical damage of the battery, but, under certain conditions, also during normal operation. A special problem in this context is the formation of dendrites at an electrode during charge; these pine crystals may pierce the separator of the battery and cause a short circuit with an adjacent counter electrode.
Thus, battery manufacturers try to control, by electronic, mechanic or chemical means, the charge/discharge circuit in such a manner that the current flow is interrupted below a critical temperature, making the occurrence of a xe2x80x9cthermal runwayxe2x80x9d impossible. To this end, e.g., pressure-sensible mechanic or temperature-sensible electronic switches are integrated into the internal battery circuit. Furthermore, it is being discussed to interrupt the current transport irreversibly by chemical reactions in the electrolyte or by mechanical changes of the separator, as soon as a critical temperature threshold is reached. Finally, it is general practice to prescribe the use of exactly specified electronic charging devices, thus strictly limiting the charge currents and the end-of-charge voltages.
However, in spite of these measures, the safety standard for many nonaqueous cells is insufficient. Lithium ion cells, for example, are only used with capacities of at most 1.3 Ah, because with bigger cells the safety risks are, based on the current state of the art, too high.
It is an object of the instant invention to increase the security of nonaqueous electrochemical cells in a manner which is as cost-effective and simple as possible.
This object is achieved by a non-aqueous electrochemical cell comprising a negative electrode, an electrolyte solution and a positive electrode which is characterized by the fact that it contains a salt in solid state in the range of at least one of the electrodes.
The invention results in a substantial improvement of operational safety. The speed of reaction in case of an internal short-circuit, is dramatically reduced. Also the building up of pressure in the cell housing is reduced drastically, thus minimizing the risk of the discharge of solid or gaseous matters or even fire. In many cases additional positive effects are obtained. In particular, a substantial reduction of self-discharge during cell storage was observed in experiments.
The causes of the advantageous effects of the invention are not yet fully clarified. It is assumed that the solid salt arranged in the vicinity of the electrode delays the access of the electrolyte to the electrode surface, thus slowing down safety-critical reactions between the electrolyte and the substances generated or deposited at the electrode surface. Also spreading of a local heating is reduced by the salt. Furthermore, it can be assumed that the salt melts in case of a strong local temperature rise. The necessary heat of fusion is extracted from the reaction. It is assumed that this effect occurs already at temperatures below the melting point of the pure salt, because salt mixtures having a lower melting point are generated by the electrochemical reactions occurring in the cell and by additional reactions occurring due to local short circuits. Furthermore, based on the experimental validation carried out so far, it can be assumed that, due to the presence of the solid salt in the range of the electrode, chemical reactions occur in case of a short-circuit which shift the chemical equilibrium of the mentioned exothermic reactions, causing them to occur only to a very reduced extent.
The increased security achieved by the invention is of special importance for secondary cells, because of the safety risks existing during the charge process. However, also for primary cells the invention creates positive effects.
A particularly important field of application are cells in which the negative electrode (anode) contains, in the charged state, a metal as active mass. Of particular importance are cells the active metal of which is an alkali metal, an alkaline earth metal or a metal of the second subgroup of the periodic chart of the elements (in particular, zinc or aluminum). Lithium, sodium and calcium cells bear particular safety risks, as they contain, in charged state, a highly reactive active metal at the anode. In case of lithium ion accumulators, for example, lithium is incorporated into an electrode which consists of graphite or of a carbonaceous compound. The electrolyte of these cells is based on an organic solvent. These components may react violently in case of external heat stress or due to sudden heat caused by a short circuit.
An important field of application of the invention are cells in which the negative electrode contains an active metal in charged state, in which an electrolyte based on sulfur dioxide is used, and in which ions of the active metal are incorporated into the positive electrode during the discharge of the cell. During the charge process of such cells, the active metal is deposited onto the negative electrode. The term xe2x80x9cSO2-based electrolytexe2x80x9d designates those electrolytes which contain SO2 not only as addition in low concentration, but in which the mobility of a species containing the active metal is based on the presence of the SO2, in other words cells in which the SO2 acts as a solvent for the species transporting the charge in the electrolyte, i.e. the ions of the active metal. Such cells can be completely inorganic, meaning that they contain neither carbon nor any organic components.
Among such cells, those in which the active metal of the negative electrode is an alkali metal, in particular lithium or sodium, are of particular practical importance. In this case, the charge-transporting species in the electrolyte is generally formed by ions of a conducting salt, preferably a tetrachloroaluminate of the alkali metal, e.g. LiAlCl4. Particularly advantageous are cells the positive electrode of which comprises a metal oxide, in particular an intercalation compound. Such a cell is described in U.S. Pat. No. 5,213,914.
In order to achieve its positive effect, the solid salt must be arranged xe2x80x9cin the rangexe2x80x9d of the electrode in such a manner, that it has an effect on the exothermic reactions occurring in the immediate vicinity of the electrode surface during safety-critical situations. Preferably, the salt is an alkali metal halide, in particular, LiF, NaCl or LiCl. An immediate contact between the electrode and the salt is not absolutely necessary. It is, however generally preferred if there is a contact to the electrode, in particular to an active mass formed at the electrode, during at least a part of the charge/discharge cycle of the cell.
Preferably, the salt should have a porous structure. A porous structure of the salt is particularly advantageous for cells in which during charge or discharge a deposit of an active mass takes place, i.e. the volume of the active mass increases at the electrode, in the area of which the solid salt is located. In such cells, the porous structure of the salt should be formed and arranged in such a manner, that the active mass which grows at the electrode during charge or discharge of the cell, penetrates into the pores of the porous salt structure. Thereby a large surface contact is provided between the salt and substances which are formed at the electrode during the charge or discharge of the cell and which may lead to safety-critical states, or which may cause generation of heat in case of a xe2x80x9cthermal runwayxe2x80x9d. This contact at a large surface area increases the efficiency of the salt.